The present application is based on and claims priority from the following Japanese Patent Applications: 2000-277074, filed Sep. 12, 2000; 2000-323201, filed Oct. 23, 2000; and 2000-394893, filed Dec. 26, 2000; the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method of manufacturing a stator yoke of a rotary electric machine.
2. Description of the Related Art
Japanese Patent application JP-A-64-60247 discloses a method of manufacturing a yoke. In the method, a sheet made of magnetic material, such as iron or steel, is rolled and a number of convexities formed at one side thereof are fitted to the same number of concavities. Thereafter, respective central portions of the convexities of one side of the steel sheet are punched and expanded, thereby clamping the opposite sides of the steel sheet. It is also known that the convexities are shaped into dovetail convexities.
However, because the convexities on a side of a steel sheet are expanded and deformed while the concavities on the other side are not expanded, differences in shape and thickness arise between the one side and the other side. This may cause the circularity of the yoke to be inaccurate. If the circularity is not accurate, it is not possible for a rotary electric machine to provide a small air gap between the rotor and the stator thereof. If the air gap is not made small, the performance of a rotary electric machine lowers.
As a conventional technology of manufacturing a stator yoke of a rotary electric machine, there is a method of manufacturing a yoke disclosed in JP-A-5-91700. In this method, a sheet member is rolled to meet the opposite sides of the sheet member together, and the opposite sides are welded together. For this purpose, positioning concavities are formed on the surface of the sheet member. However, an expensive laser beam has to be used to weld the opposite sides.
It is also known that dovetail convexities are formed at one side of the sheet member, which are fitted and clamped to concavities formed at the other side of the sheet member. In this case, sealant is filled in the clamped portions to ensure the seal, as disclosed in JP-A-52-20207. However, extra steps of filling sealant and removing leftover sealant are necessary, and work time and production cost increase.
Further, a yoke of a motor, such as a starter motor, is manufactured from a steel plate, which is rolled up to form a cylindrical shape so that the opposite sides of the plate are put together and welded to each other as disclosed in JP-U-58-97957 and JP-A-64-60247. There is a chamfered corner at the inside of the yoke in order to have a bracket or the like fitted in the open end of the yoke smoothly.
Such chamfered corner of the yoke is formed by a machine after the steel plate is rolled up. Therefore, the manufacturing cost is high.
A main object of the present invention is to provide a method of manufacturing a yoke having an accurate circularity.
A main feature of the invention is a step of clamping in which the border portions of both dovetail convexities are punched.
It is only necessary for the first dovetail convexities and the second dovetail convexities to have a wider tail end than a tail base so that it can fit to each other. It is not necessary for them to have a trapezoidal shape. It is not necessary that the step of cutting a steel sheet, the step of fitting the dovetail convexities and the step of clamping the dovetail convexities are separately carried out. Further, it is not necessary that machines for respective steps are separate from each other. It is also possible to integrate such machines so that they can carry out a series of the steps successively.
Because the punches are given along the border of both the first and second dovetail convexities to expand uniformly in the clamping step, both sides of the steel sheet are deformed to the same degree. Therefore, a simple and inexpensive method that is comprised of the step of rolling a steel sheet, the step of meeting the opposite sides thereof together and the step of clamping them can improve the circularity of the yoke. Further, a cylindrical yoke having an accurate circularity can be manufactured from a steel sheet at a comparatively low cost. In addition, the first and second dovetail convexities are respectively formed at almost the whole length of the opposite sides thereof and clamped. Therefore, the fastening strength is increased.
According to another feature of the invention, both the first and second dovetail convexities are the same in size and shape. They are given punches on the border equally, and they are deformed equally so that a very accurate circularity of the yoke can be provided.
According to another feature of the invention, the first and second dovetail convexities are given punches only on the middle border portions where a straight line crosses, and the clamping can be completed by a smallest number of punches. In addition, work time of the clamping can be reduced because the positions on which the punches are given are on a straight line.
Therefore, the manufacturing cost and work time can be reduced.
According to another feature of the invention, punches are given on the middle of the longitudinal borderline to clamp the opposite sides of the rolled steel sheet. Therefore, compressed stress or bending strain does not appear at the axial ends of the borderline, and the opposite sides of the rolled steel sheet are well prevented from separating from each other at the axial ends of the yoke.
According to another feature of the invention, the profile of the first and second dovetail convexities includes a straight portion. Therefore, the steel sheet can be cut easily at a low cost. In other words, since the dies of the press machine includes the corresponding straight portion, the dies can be made at a low cost. Therefore, work time and the cost of production can be reduced.
Another object of the present invention is to provide a simple method of manufacturing a well-sealed yoke at a low cost.
According to a feature of the invention for this object, an outer periphery of a yoke is electro-statically painted in a painting step to seal portions where convexities and concavities are fitted to each other. Since the outer periphery of the yoke is electro-statically painted, small gaps can be sealed by paint. Therefore, a sufficient hermetic seal can be provided. Since the surface of the clamped portion sealed by the electro-static painting is smooth, no step of smoothing the yoke surface is necessary.
According to another feature of the invention, a gap opening to the outer periphery is formed at least the clamped portions. Therefore, electric charge gathers around the gap when the peripheral surface of the yoke is electro-statically painted, and the paint moving to the clamped portion gets into the gap.
Therefore, a very smooth peripheral surface of the yoke can be provided.
According to another feature of the invention, the gap is less than twice as thick as a paint film formed in the painting step, the gap can be sufficiently filled with the paint.
Accordingly, the yield rate of the paint can be improved and the production cost and time can be reduced.
According to another feature of the invention, the yoke is heated before powder paint is sprayed on the peripheral surface, the paint can stick to the peripheral surface very well. Therefore, a beautiful yoke can be provided.
According to another feature of the invention, the starting point of the painting and the ending point of the painting are set at the clamped portion so that the clamped portion can be painted twice. Therefore, the gaps at the clamped portion can be sufficiently filled with paint, and a higher hermetic yoke can be provided without additional cost.
Another object of the invention is to provide a yoke of a rotary electric machine that can eliminate the machine work for the chamfering.
A yoke of a motor is formed from a steel plate that is rolled to form a cylindrical shape having axially opposite ends that are chamfered at inside or outside corners thereof. According to a feature of the invention, the steel plate is chamfered before it is rolled up.
This invention omits additional chamfering machine work. If a member (e.g. a bracket) to be assembled to the yoke is fitted to the inside of the yoke in the axial direction thereof, the inside corner of the yoke is chamfered. On the other hand, the outside corner of the yoke is chamfered if the member is fitted to the outside of the yoke, so that cracking from the shear drop can be prevented.
According to another feature of the invention, the steel plate is cut by a press machine into a strip having a fixed width and rolled so as to locate a shear drop that is formed due to cutting by a press machine at the outside of said cylindrical shape. Accordingly, the cut surface does not expand circumferentially outer direction when the steel plate is rolled up so that cracking can be prevented from extending from the cut surface.